This invention relates to a method of attenuating the echo signal in converting circuits, referred to as telephone fork circuits, intended for telephone conferences held with an odd number of participants, said circuits being useful for matching double-wire telephone lines, each having analog phonic messages transmitted bidirectionally thereover to and from each of the participants, and pairs of double-wire lines, with each line in each pair being respectively adapted to receive and transmit said messages as converted each from and into a digital code of the PCM (Pulse Code Modulation) type.
The invention is also concerned with an apparatus operating in accordance with the above-noted method.
The field of application of the method and apparatus according to the invention comprehends in particular, though not solely, digital switching telephone exchanges and private telephone branch exchanges, also referred to as digital PABX (Private Automatic Branch Exchange), wherein there exists a demand for a conference switching phase between messages, coded according to the PCM digital code, from corresponding participants in the conference.
As is known, a telephone conference between a number of participants equipped with a corresponding subscriber telephone set is currently held, according to prior art approaches, with the intermediary of a pair of integrated electronic components consisting of a digital switching array cooperating with a conference actuator, both components being controlled by means of a microprocessor.
In particular a switching array, commercially available as SGS MO88, receives a number of channels each carrying a PCM-coded message, and performs switching between input and output channels.
Such input and output channels are concentrated on serial lines, each capable of being allocated up to thirty-two discrete channels by a conventional technique referred to as TDM (Time Division Multiplexing).
The second-mentioned of said component pair, commercially available as SGS M116, consists of a conference actuator which receives sequentially at its input, from a serial line, the PCM-coded messages supplied by the switching array, and performs arithmetic summing operations on the PCM coding of such messages, thus altering the value of the analog signal to which the coding applies.
Each of the participants in the conference, being equipped with a corresponding subscriber telephone set, is located at one end of a double-wire telephone line adapted for bidirectional transmission of analog phonic messages from and to each participant.
At the telephone exchange, the other end of the double-wire line is connected to a converting or interface circuit, referred to as telephone fork or "hybrid" circuit, which interfaces said line to a pair of discrete double-wire lines, respectively and separately operative to transmit and receive messages.
Message messages or signals designated ST and RT respectively are transmitted or received from and to a participant in the conference on a corresponding one of the aforesaid line pairs.
Further, the line intended for the transmitted signal ST is connected to an analog-to-digital converter whose output is multiplexed by the aforesaid TDM technique with the equivalent outputs of another thirty-one analog-to-digital converters on a single serial line, thus issuing a multiplexed digital signal which contains thirty-two PCM messages related to as many subscribers.
That serial line is applied to one of eight inputs to the switching array, which can therefore receive two hundred fifty-six discrete PCM messages. In a dual mode, the switching array has eight outputs, each having thirty-two PCM messages multiplexed thereon. Each of these outputs can be connected to a buffer which, using the same TDM technique, will read out the individual PCM messages and pass them to the inputs of thirty-two digital-to-analog converters whose outputs are applied to the respective RT signal receiving lines.
This prior approach has, however, a serious drawback in that on the transmission line there appears a signal component, referred to as echo, which is proportional to the receiving line signal.
In particular, on the transition from two to four wires through the telephone fork converting circuit, the transmitted signal ST is equal to the signal S transmitted on the double-wire line, connected directly to the participant in the conference, plus a signal component given by the product of the received signal RT multiplied by a parameter K. The value of this parameter K is variable and tied to the impedance Z of the double-wire telephone line interconnecting the participant in the conference and the converting circuit, as well as to the telephone signal frequency within the range of 300 to 3400 Hz.
The term KRT constitutes the aforesaid echo signal that affects and deteriorates the conversations of the participants in the conference.